Between the years of 4,000 BBY and the millennia leading up to the Imperial Era, the Gungan leader Boss Gallo assembled a substantial military force during a period of war among the Gungan tribes of the planet Naboo. Gallo strategically utilized this army, coupled with diplomatic efforts, to bring an end to the conflict. Ultimately, he deployed it to destroy the city of Spearhead, which served as the stronghold of the renegade Boss Rogoe and the remaining Gungan resistance. In the aftermath, Gallo established Otoh Gunga, the new Gungan capital, on the location where Spearhead once stood.
Within current Star Wars canon, Gallo's military force received a mention in Nexus of Power. This is a 2016 sourcebook created by Fantasy Flight Games for the Star Wars: Force and Destiny roleplaying game. Shifting to the Star Wars Legends continuity, the forces under Gallo's command were initially referenced in Star Wars: Galactic Battlegrounds: Prima's Official Strategy Guide by Steven L. Kent, which was published on November 7, 2001.
Subsequently, the army made an appearance in the LucasArts video game titled Star Wars: Galactic Battlegrounds, which was released on November 11 of the same year. This game identified the force as a preliminary version of the Gungan Grand Army. The Gungan Grand Army had been conceived for the prequel trilogy film Star Wars: Episode I The Phantom Menace, which premiered on May 19, 1999. However, its initial appearance was in the film's novelization penned by Terry Brooks, which was available from April 21, 1999. It is important to note that Gallo's army has not been connected to the Gungan Grand Army within the current canon.